There are many fields in which the liquid being used must be deaerated, such as the operation of analytical equipment (e.g., liquid chromatography, automatic chemical analysis for clinical use, and spectrophotometers for medical use) and various industrial processes (e.g., deionization of water, ultrapure water systems, water for steam boilers, water for nuclear power plants, and water for turbines).
In liquid chromatogrphy, for example, dissolved air, if present in the solvent, will form bubbles in the pump, around the valves and in the detector, causing various troubles. In addition, dissolved oxygen can cause chemical reactions with some of the solutes. In automatic chemical analysis for clinical use, in which the amount of the test sample tends to become smaller in recent years, the measurement accuracy is adversely affected even by a small amount of dissolved oxygen. In the use of a spectrophotometer, absorption is greatly affected by dissolved oxygen and other gases at shorter wavelengths in the ultraviolet region. In the process of deionizing water, on the other hand, oxygen or carbon dioxide dissolved in the liquid shortens the service life of ion-exchange resin. In steam boilers and nuclear power plants, oxygen dissolved in water accelerates the corrosion of vessels and pipes.
Heat boiling, evacuation, ultrasonication and helium methods have hitherto been used for deaeration of liquid. However, each of these methods is not satisfactory in terms of effect and cost; the heat boiling method is highly dangerous because of the high-temperature operations, the evacuation and aultrasonication methods have a low ability to remove dissolved gases, and the helium method suffers from high operation costs.
To put it in more detail, gases dissolved in water to be supplied to steam boilers (particularly dissolved oxygen) can be a primary cause of printing corrosion in boilers and preboilers, and hence must be completely removed prior to use. Two types of methods are known to remove dissolved oxygen in these systems: the methods of mechanically removing oxygen by the use of a deaerator (the heat boiling and evacuation the methods), and methods of chemically reducing dissolved oxygen by the use of a deoxidizing agent, such as hydrazine and sodium sulfite. A high deoxidization efficiency can be achieved only when these two types of methods are used in combination, particularly with the water to be supplied to medium- and high-pressure steam boilers.
Gases dissolved in drinking water and water to be supplied to office buildings particularly dissolved oxygen ) are primarily responsible for the corrosion of feed pipes, which lead to the so-called "reddish water". Reddish water causes sensory problems (e.g., disagreeable taste) and discoloration of washing; hence, some actions are taken when its generation is observed, such as replacement of feed pipes with new ones, renewal of feed pipes by lining, and continuous injection of a rust preventive for feed water. However, any of these actions is not satisfactory in terms of cost, effectiveness or safety, and is accompanied by various restrictions. Under the circumstances, there has been a demand for a simple, effective and low-cost method of preventing such troubles.
The objects of deaeration in ultrapure water systems may be roughly divided into two types: one is to remove dissolved carbon dioxide in order to elongate the service life of anion-exchange resins used in the ion-exchange process, and the other is to remove dissolved oxygen gas from the produced ultrapure water in order to prevent the propagation of microbial cells therein. In the manufacture of semiconductors, the concentration of dissolved oxygen (hereinafter referred to as the OD value) may be a level of 0.5 ppm or lower to achieve the above objects with a memory capacity up to 256 Kbit. Hence, the evacuation method has been used for this purpose.
In recent years, however, semiconductors with a memory capacity of 4 Mbit and 16 Mbit have been developed, for which deaeration is also intended (in addition to the above-mentioned objects) to prevent the formation of an oxide layer on the surface of silicon wafer by dissolved oxygen. To achieve this object, the OD value must be in the range from 0.01 to 0.05 ppm. Furthermore, use of a relatively small size deaerator is required because deaeration must be carried out around the use point. Vacuum deaerators conventionally used are insufficient in deoxidizing ability and are rather large in size.
The water used for the manufacture of beverages (e.g., beer, fruit juices and coffee) should preferably be free of dissolved oxygen and germfree to prevent deterioration and oxidation of the final products.
Such water was formerly produced by the heat boiling method, the evacuation method or the method of bringing carbon dioxide or a mixture thereof with an inert gas into contact with the water being treated.
In the heat boiling method, the water must be heated at temperature of 104.degree. C. or higher in order to reduce the OD value to about 0.1 ppm. The result is a high energy cost, and deposition of scale in various parts of the equipment after long period of operation, which requires much labor to wash it off. The evacuation method is low in deaeration ability, being capable of reducing the OD value only to about 0.2 ppm. In the method of bringing carbon dioxide gas into contact with the water being treated, it is essential to charge the equipment with packings, such as Raschig rings, and to maintain the temperature at about 70.degree. C. in order to reduce the OD value to about 0.1 ppm. Much labor is needed to wash the packings, and the treated water contains dissolved carbon dioxide gas and hence fails to give delicious products when used to brew coffee and like drinks. In the method of using a mixture of carbon dioxide and an inert gas, the water being treated must be heated at a temperature of 101.degree. C. or higher in order to reduce the OD value to about 0.05 ppm, resulting in a high energy cost. Furthermore, delicious coffee and like drinks cannot be obtained in this case too.
As stated above, there are many fields requiring deaeration, and no satisfactory method has yet been established in any of these fields.
A method of deaeration using a tubular membrane made of a synthetic resin, such as silicone resins and polytetrafluoroethylene (Japanese Patent Kokai No.25514/1985, Japanese Utility Model Kokai No.43609/1988, etc.), has been recently proposed.
These resinous membranes, however, are limited in mechanical strengths and moldability, and also have the problem that membranes of larger thickness must be used in cases of lower deaeration rates.